The significance of electrostatic discharge ("ESD") is one of the ironies of the semiconductor industry. As semiconductor devices become denser, higher performing, and generally more pervasive in our culture, so too do they become more sensitive to the seemingly insignificant threat of ESD. A thumbnail size piece of silicon containing a million or more transistors may be destroyed by the electrostatic charge accumulated on the soles of a careless assembly technician.
ESD protection devices are incorporated at each input/output path in most semiconductor devices to shunt excessive charge away from sensitive circuits. These devices may be separate circuits or may be incorporated into an adjacent input/output buffer. Regardless of where they are located, ESD protection devices are relatively large circuits. The combination of their size and number often adversely impacts the total size of the integrated circuit of which they are a part. Consequently, designers take every opportunity to reduce the effective size of their ESD protection devices.